The present invention relates to a process for producing a catalyst for the catalytic cracking of olefins, in particular a cracking process which is selective towards light olefins in the effluent. In particular by using such a catalyst, olefinic feedstock from refineries or petrochemical plants can be converted selectively so as to redistribute the olefin content of the feedstock in the resultant effluent.
It is known in the art to use zeolites to convert long chain paraffins into lighter products, for example in the catalytic dewaxing of petroleum feedstocks. While it is not the objective of dewaxing, at least parts of the paraffinic hydrocarbons are converted into olefins. It is known in such processes to use crystalline silicates for example of the MFI type, the three-letter designation xe2x80x9cMFIxe2x80x9d representing a particular crystalline silicate structure type as established by the Structure Commission of the International Zeolite Association. Examples of a crystalline silicate of the MFI type are the synthetic zeolite ZSM-5 and silicalite and other MFI type crystalline silicates are known in the art.
GB-A-1323710 discloses a dewaxing process for the removal of straight-chain paraffins and slightly branched-chain paraffins, from hydrocarbon feedstocks utilising a crystalline silicate catalyst, in particular ZSM-5. U.S. Pat. No. 4,247,388 also discloses a method of catalytic hydrodewaxing of petroleum and synthetic hydrocarbon feedstocks using a crystalline silicate of the ZSM-5 type. Similar dewaxing processes are disclosed in U.S. Pat. No. 4,284,529 and U.S. Pat. No. 5,614,079. The catalysts are crystalline alumino-silicates and the above-identified prior art documents disclose the use of a wide range of Si/Al ratios and differing reaction conditions for the disclosed dewaxing processes.
GB-A-2185753 discloses the dewaxing of hydrocarbon feedstocks using a silicalite catalyst. U.S. Pat. No. 4,394,251 discloses hydrocarbon conversion with a crystalline silicate particle having an aluminum-containing outer shell.
It is also known in the art to effect selective conversion of hydrocarbon feeds containing straight-chain and/or slightly branched-chain hydrocarbons, in particular paraffins, into a lower molecular weight product mixture containing a significant amount of olefins. The conversion is effected by contacting the feed with a crystalline silicate known as silicalite, as disclosed in GB-A-2075045, U.S. Pat. No. 4,401,555 and U.S. Pat. No. 4,309,276. Silicalite is disclosed in U.S. Pat. No. 4,061,724.
Silicalite catalysts exist having varying silicon/aluminum atomic ratios and different crystalline forms. EP-A-0146524 and 0146525 in the name of Cosden Technology, Inc. disclose crystalline silicas of the silicalite type having monoclinic symmetry and a process for their preparation. These silicates have a silicon to aluminum atomic ratio of greater than 80.
WO-A-97/04871 discloses the treatment of a medium pore zeolite with steam followed by treatment with an acidic solution for improving the butene selectivity of the zeolite in catalytic cracking.
A paper entitled xe2x80x9cDe-alumination of HZSM-5 zeolites: Effect of steaming on acidity and aromatization activityxe2x80x9d, de Lucas et al, Applied Catalysis A: General 154 1997 221-240, published by Elsevier Science B.V. discloses the conversion of acetone/n-butanol mixtures to hydrocarbons over such dealuminated zeolites.
U.S. Pat. No. 5,043,307 discloses a modified crystalline alumino-silicate zeolite catalyst and its use in the production of lubes of high viscosity index. The catalyst is modified by use of a process in which an as synthesised crystalline aluminosilicate containing organic template material is steamed to decompose at least a portion of the template material and to extract aluminium from the zeolite. The zeolite is then contacted with a dealuminising agent which forms a water-soluble complex with aluminum to remove a further quantity of zeolitic aluminium from the zeolite. Since the zeolite contains the organic template, only the surface of the zeolite is so modified. The surface-modified zeolite (such as ZSM-5) has a silica/alumina ratio of up to 108. The catalyst is not disclosed as having utility in the catalytic cracking of olefins, in particular for the production of propylene.
It is yet further known, for example from U.S. Pat. No. 4,171,257, to dewax petroleum distillates using a crystalline silicate catalyst such as ZSM-5 to produce a light olefin fraction, for example a C3 to C4 olefin fraction. Typically, the reactor temperature reaches around 500xc2x0 C. and the reactor employs a low hydrocarbon partial pressure which favours the conversion of the petroleum distillates into propylene. Dewaxing cracks paraffinic chains leading to a decrease in the viscosity of the feedstock distillates, but also yields a minor production of olefins from the cracked paraffins.
EP-A-0305720 discloses the production of gaseous olefins by catalytic conversion of hydrocarbons. EP-B-0347003 discloses a process for the conversion of a hydrocarbonaceous feedstock into light olefins. WO-A-90/11338 discloses a process for the conversion of C2-C12 paraffinic hydrocarbons to petrochemical feedstocks, in particular to C2 to C4 olefins. U.S. Pat. No. 5,043,522 and EP-A-0395345 disclose the production of olefins from paraffins having four or more carbon atoms. EP-A-0511013 discloses the production of olefins from hydrocarbons using a steam activated catalyst containing phosphorous and H-ZSM-5. U.S. Pat. No. 4,810,356 discloses a process for the treatment of gas oils by dewaxing over a silicalite catalyst. GB-A-2156845 discloses the production of isobutylene from propylene or a mixture of hydrocarbons containing propylene. GB-A-2159833 discloses the production of a isobutylene by the catalytic cracking of light distillates.
It is known in the art that for the crystalline silicates exemplified above, long chain olefins tend to crack at a much higher rate than the corresponding long chain paraffins.
It is further known that when crystalline silicates are employed as catalysts for the conversion of paraffins into olefins, such conversion is not stable against time. The conversion rate decreases as the time on stream increases, which is due to formation of coke (carbon) which is deposited on the catalyst.
These known processes are employed to crack heavy paraffinic molecules into lighter molecules. However, when it is desired to produce propylene, not only are the yields low but also the stability of the crystalline silicate catalyst is low. For example, in an FCC unit a typical propylene output is 3.5 wt % The propylene output may be increased to up to about 7-8 wt % propylene from the FCC unit by introducing the known ZSM-5 catalyst into the FCC unit to xe2x80x9csqueezexe2x80x9d out more propylene from the incoming hydrocarbon feedstock being cracked. Not only is this increase in yield quite small, but also the ZSM-5 catalyst has low stability in the FCC unit.
There is an increasing demand for propylene in particular for the manufacture of polypropylene.
The petrochemical industry is presently facing a major squeeze in propylene availability as a result of the growth in propylene derivatives, especially polypropylene. Traditional methods to increase propylene production are not entirely satisfactory. For example, additional naphtha steam cracking units which produce about twice as much ethylene as propylene are an expensive way to yield propylene since the feedstock is valuable and the capital investment is very high. Naphtha is in competition as a feedstock for steam crackers because it is a base for the production of gasoline in the refinery. Propane dehydrogenation gives a high yield of propylene but the feedstock (propane) is only cost effective during limited periods of the year, making the process expensive and limiting the production of propylene. Propylene is obtained from FCC units but at a relatively low yield and increasing the yield has proven to be expensive and limited. Yet another route known as metathesis or disproportionation enables the production of propylene from ethylene and butene. Often, combined with a steam cracker, this technology is expensive since it uses ethylene as a feedstock which is at least as valuable as propylene.
Thus there is a need for a high yield propylene production method which can readily be integrated into a refinery or petrochemical plant, taking advantage of feedstocks that are less valuable for the market place (having few alternatives on the market).
On the other hand, crystalline silicates of the MFI type are also well known catalysts for the oligomerisation of olefins. For example, EP-A-0031675 discloses the conversion of olefin-containing mixtures to gasoline over a catalyst such as ZSM-5. As will be apparent to a person skilled in the art, the operating conditions for the oligomerisation reaction differ significantly from those used for cracking. Typically, in the oligomerisation reactor the temperature does not exceed around 400xc2x0 C. and a high pressure favours the oligomerisation reactions.
GB-A-2156844 discloses a process for the isomerisation of olefins over silicalite as a catalyst. U.S. Pat. No. 4,579,989 discloses the conversion of olefins to higher molecular weight hydrocarbons over a silicalite catalyst. U.S. Pat. No. 4,746,762 discloses the upgrading of light olefins to produce hydrocarbons rich in C5+ liquids over a crystalline silicate catalyst. U.S. Pat. No. 5,004,852 discloses a two-stage process for conversion of olefins to high octane gasoline wherein in the first stage olefins are oligomerised to C5+ olefins. U.S. Pat. No. 5,171,331 discloses a process for the production of gasoline comprising oligomerising a C2-C6 olefin containing feedstock over an intermediate pore size siliceous crystalline molecular sieve catalyst such as silicalite, halogen stabilised silicalite or a zeolite. U.S. Pat. No. 4,414,423 discloses a multistep process for preparing high-boiling hydrocarbons from normally gaseous hydrocarbons, the first step comprising feeding normally gaseous olefins over an intermediate pore size siliceous crystalline molecular sieve catalyst. U.S. Pat. No. 4,417,088 discloses the dimerising and trimerising of high carbon olefins over silicalite. U.S. Pat. No. 4,417,086 discloses an oligomerisation process for olefins over silicalite. GB-A-2106131 and GB-A-2106132 disclose the oligomerisation of olefins over catalysts such as zeolite or silicalite to produce high boiling hydrocarbons. GB-A-2106533 discloses the oligomerisation of gaseous olefins over zeolite or silicalite.
It is an object of the present invention to provide a process for producing a catalyst which is for use in a cracking process employing the less valuable olefins present in refinery and petrochemical plants as a feedstock for the cracking process which, in contrast to the prior art processes referred to above, catalytically converts olefins into lighter olefins, and in particular propylene.
It is another object of the invention to provide a catalyst for use in a process for producing propylene having a high propylene yield and purity.
It is a further object of the present invention to provide a catalyst for use in such a process which can produce olefin effluents which are within, at least, a chemical grade quality.
It is yet a further object of the present invention to provide a catalyst for use in a process for producing olefins having a stable olefinic conversion and a stable product distribution over time.
It is yet a further object of the present invention to provide a catalyst for use in a process for converting olefinic feedstocks having a high yield on an olefin basis towards propylene, irrespective of the origin and composition of the olefinic feedstock.
It is still a further object of the invention to provide a process for producing a catalyst for olefin cracking having high stability, for example capable of giving a stable selected olefin yield over a significant period of time, typically several days.
It is another object to provide such a catalyst which can be regenerated several times.
It is still another object to provide such a catalyst which has high flexibility so that it can operate with a variety of different feedstocks, which may be pure or mixtures.
The present invention provides a process for producing a catalyst for olefin cracking, the process comprising the steps of providing an MFI-type crystalline silicate catalyst, heating the catalyst in steam to remove aluminum from the crystalline silicate framework and extracting aluminum from the catalyst by contacting the catalyst with a complexing agent for aluminum to remove from pores of the framework aluminum deposited therein during the steaming step thereby to increase the silicon/aluminum atomic ratio of the catalyst; and calcining the catalyst at elevated temperature.
The present invention further provides a process for the catalytic cracking of olefins in a hydrocarbon feedstock which is selective towards light olefins in the effluent, the process comprising contacting a hydrocarbon feedstock containing olefins having a first composition of one or more olefinic components with a catalyst produced in accordance with the process of the invention to produce an effluent having a second composition of one or more olefinic components, with the feedstock and the effluent having substantially the same olefin content by weight therein.
The present invention yet further provides the use of a MFI-type crystalline silicate catalyst produced in accordance with the invention for cracking an olefin-rich hydrocarbon feedstock, which cracking is selective towards propylene in the effluent.
The present invention can thus provide a process wherein olefin-rich hydrocarbon streams (products) from refinery and petrochemical plants are selectively cracked not only into light olefins, but particularly into propylene. The olefin-rich feedstock may be passed over a crystalline silicate catalyst with a particular Si/Al atomic ratio of at least 180 obtained after a steaming/de-alumination treatment. The feedstock may be passed over the catalyst at a temperature ranging between 500 to 600xc2x0 C., an olefin partial pressure of from 0.1 to 2 bars and an LHSV of from 10 to 30 hxe2x88x921 to yield at least 30 to 50% propylene based on the olefin content in the feedstock.
In this specification, the term xe2x80x9csilicon/aluminum atomic ratioxe2x80x9d is intended to mean the Si/Al atomic ratio of the overall material, which may be determined by chemical analysis. In particular, for crystalline silicate materials, the stated Si/Al ratios apply not just to the Si/Al framework of the crystalline silicate but rather to the whole material.
The silicon/aluminum atomic ratio is preferably greater than about 180. Even at silicon/aluminum atomic ratios less than about 180, the yield of light olefins, in particular propylene, as a result of the catalytic cracking of the olefin-rich feedstock may be greater than in the prior art processes. The feedstock may be fed either undiluted or diluted with an inert gas such as nitrogen. In the latter case, the absolute pressure of the feedstock constitutes the partial pressure of the hydrocarbon feedstock in the inert gas.